Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for testing an accelerometer to be used for seismic data acquisition.
Discussion of the Background
Marine seismic data acquisition and processing generate a profile (image) of the geophysical structure (subsurface) under the seafloor. While this profile does not provide an accurate location for the oil and gas reservoirs, it suggests, to those trained in the field, the presence or absence of reservoirs. Thus, providing a high-resolution image of the subsurface is an ongoing process for the exploration of natural resources, including, among others, oil and/or gas.
During a seismic gathering process, a seismic survey system 100, as shown in FIG. 1, includes a vessel 102 that tows plural receivers 104 distributed along a streamer 106. Vessel 102 may tow plural streamers 106 at the same time. The streamers may be disposed horizontally, i.e., lying at a constant depth z1 relative to the ocean surface 110. Also, the plural streamers 116 may form a constant angle (i.e., the streamers may be slanted) with respect to the ocean surface as disclosed in U.S. Pat. No. 4,992,992, the entire content of which is incorporated herein by reference. Alternatively, the streamers may have a variable-depth profile, as described in U.S. patent application Ser. No. 13/464,149, the entire content of which is incorporated herein by reference.
Still with reference to FIG. 1, each streamer may have a head float 106a and a tail buoy 106b connected to its respective ends for maintaining given depth z1. A front-end gear 112 that includes various cables connects streamers 106 to vessel 102. Vessel 102 also tows a sound source 120 configured to generate an acoustic wave 122a. Acoustic wave 122a propagates downward and penetrates the seafloor 124, eventually being reflected by a reflecting structure 126 (reflector). The reflected acoustic wave 122b propagates upwardly and is detected by detector 104. For simplicity, FIG. 1 shows only one path 122a corresponding to the acoustic wave. However, the acoustic wave emitted by source 120 may be a substantially spherical wave, e.g., it propagates in all directions starting from source 120. Parts of reflected acoustic wave 122b (primary) are recorded by the various sensors 104 (recorded signals are called traces) while parts 122c of reflected wave 122b pass the sensors 104 and arrive at the water surface 110. Since the interface between the water and air is well approximated as a quasi-perfect reflector (i.e., the water surface acts as a mirror for acoustic waves), reflected wave 122c is reflected back toward another detector 104 as shown by wave 122d in FIG. 1. Wave 122d is normally referred to as a ghost wave because it is due to a spurious reflection. Ghosts are also recorded by sensors 104, but with a reverse polarity and a time lag relative to primary wave 122b. The ghost's degenerative effects upon arrival on seismic bandwidth and resolution are known. In essence, interference between primary and ghost arrivals causes notches, or gaps, in the frequency content the detectors record, which reduces useful bandwidth.
Recorded traces (recorded with sensors 104, e.g. hydrophones that record a pressure change or accelerometers that record particle motions) may be used to determine an image of the subsurface (i.e., earth structure below surface 124). However, to produce a high-quality subsurface image, seismic sensors like motion sensors used in the streamer need to be checked to perform as designed, i.e., have actual characteristics conforming to design characteristics envisioned by the design engineer.
Thus, there is a need to have an apparatus and method that can easily and efficiently test one or more features of a sensor prior to using it in the field.